rtpjitterbuffer: Check srcresult before waiting on the condition variable too

It might've been set to FLUSHING between the last check and the waiting,
and in that case we'd be waiting here forever now.

Part-of: <gstreamer/gst-plugins-good!948>
32 jobs for !948 with backport-mr-944-into-1.18-jitterbuffer-flushing in 56 minutes and 43 seconds (queued for 6 seconds)
merge request